The research reported in this thesis comprises a part of a larger study of the structure and function of the E. coli respiratory chain. With the isolation in our laboratory of growing numbers of mutant strains with defects in one of the two terminal oxidases, the cytochrome o and cytochrome d complexes, it became important to characterize and quantify all the cytochromes in the wild-type and mutant strains. Potentiometric titrations in the presence of carbon monoxide combined with poised low-temperature difference spectroscopy clearly resolved and identified all the cytochrome species. When applied to a study of various mutant strains, this analysis revealed that mutants containing only subunit I of the two-subunit cytochrome d complex were missing cytochromes d and b(,595) of the complex, while the retained cytochrome b(,558). Electrochemical and spectroscopic characterization of the purified cytochrome d complex indicated that the cytochrome formally called "a(,1)" appears to have a b-type heme as its prosthetic group and has a resolved spectrum similar to that of peroxidases and catalases. For this reason, it has been renamed cytochrome b(,595). Coulometric analysis indicated the presence of two cytochrome d centers and one cytochrome b(,558) center per cytochrome d complex.The active sites of the purified cytochrome d complex, which serves a functional role as a ubiquinol oxidase, were probed. Four independent probes, trypsin digestion, inactivation with azido-ubiquinone derivatives, detergents, and monoclonal antibodies against subunit I, all could specifically inhibit the ubiquinol oxidase activity while unaffecting the oxidase activity using an artificial electron donor, TMPD, as substrate. Furthermore this inhibition of ubiquinol oxidation could clearly be located on subunit I. Trypsin digestion of intact spheroplasts was further used to show that the site of ubiquinol oxidation is located on the outer surface of the inner membrane.